Alpha mask help

Hi, I have this shader that I want to add an alpha mask to:

Code (CSharp):

1.  
2. Shader "Kybernetik/Simple Sun"
3. {
4.     Properties
5.     {
6.         _Color("Color", Color) = (1,1,1,1)
7.         _BallRadius("Ball Radius", Range(0, 0.5)) = 0.45
8.         _BallSharpness("Ball Sharpness", float) = 6
9.         _RaySpeed("Ray Speed", float) = 3
10.         _RayFrequency("Ray Frequency", float) = 2// Whole numbers are recommended to avoid creating a seam.
11.         _RayIntensity("Ray Intensity", float) = 100
12.         _RayRoughness("Ray Roughness", Range(0, 1.5)) = 1
13.         _Noise("Noise", 2D) = "white" {}
14.         _Alpha("Alpha (A)", 2D) = "white" {}
15.     }
16.  
17.     Category
18.     {
19.  
20.         Tags
21.         {
22.             "Queue" = "Transparent"
23.             "IgnoreProjector" = "True"
24.             "RenderType" = "Transparent"
25.             "PreviewType" = "Plane"
26.         }
27.  
28.         LOD 200
29.                
30.         Blend SrcAlpha OneMinusSrcAlpha
31.         Alphatest Greater 0
32.         ZWrite Off
33.  
34.         SubShader
35.         {
36.             Pass
37.             {
38.  
39.                 CGPROGRAM
40.        
41. ........
42.  
43.                 ENDCG
44.                     SetTexture[_MainTex]{
45.                     Combine texture
46.                 }
47.                     SetTexture[_Alpha]{
48.                     Combine previous, texture
49.                 }
50.             }
51.         }
52.     }
53.  
54.     FallBack "Diffuse"
55.  
56. }
57.  

I read around and found this settexture method (line 44-49) but my shader has no _MainTex property. Is there a way I can still put a mask in?